Saponified ethylene-vinyl ester copolymer (i.e. EVOH resin) has a very strong intermolecular force because of hydrogen bond between hydroxyl groups derived from saponified vinyl ester monomer unit. The strong intermolecular force contributes a high crystallinity to EVOH resin, and exists even in an amorphous portion. Therefore, it is impossible to permeate gas molecules or the like through an EVOH resin film. Films using EVOH resin have superior gas-barrier properties, and are used as containers for water, drinks and foods, packaging films, or packaging container materials. However, an EVOH resin molded article has a disadvantage that flexibility is poor because of high crystallinity.
In view of the above, it is generally practiced to mix EVOH resin and a flexible polymer to give flexibility to the EVOH resin molded article, when flexibility is required.
For instance, Japanese Unexamined Patent Publication No. Hei 10-87923 (Patent document 1) discloses an EVOH resin composition containing a block copolymer of a polymer block composed of vinyl aromatic monomer units and a polymer block composed of isobutylene units, as represented by a styrene-isobutylene-styrene triblock copolymer (SIBS).
It is preferable to mix EVOH resin and the block copolymer in the ratio range of 15:85 to 85:15, preferably 80:20 to 20:80, and more preferably 65:35 to 35:65 so that the EVOH resin phase and the block copolymer phase each composes a continuous phase (see paragraph 0043). Patent document 1 describes that the phase of EVOH resin and the phase of the block copolymer are distributed in an IPN (interpenetrating polymer network) structure, and therefore, the resin composition has shielding performance such as superior gas-barrier properties based on the EVOH resin phase having the network structure, and has flexibility based on the block copolymer phase which is interpenetrated in the network structure (see paragraph 0042).
Patent document 1 also discloses that gas-barrier properties may be degraded or JIS D hardness may be increased, even if the mixing ratio between EVOH resin and a triblock copolymer lies in the above range, depending on the ethylene content or the melting viscosity of EVOH resin, or the molecular weight or the constitution of a triblock copolymer (see reference examples 1 and 2 in Table 2).
Further, Japanese Unexamined Patent Publication No. Sho 63-304043 (Patent document 2) proposes use of a block copolymer modified with unsaturated carboxylic acid, in place of using an unmodified block copolymer, for the purpose of solving the difficulty in obtaining intended properties improvement, the difficulty based on unevenness resulting from poor compatibility and poor mixing between a polyvinyl alcohol-based resin such as EVOH resin, and an unmodified block copolymer such as a styrene-butadiene block copolymer or a styrene-isoprene block copolymer. Patent document 2 discloses that it is possible to improve transparency of a resin composition containing a modified block copolymer, because the modified block copolymer has superior compatibility with a polar thermoplastic resin such as a polyvinyl alcohol-based resin, as compared with an unmodified block copolymer.
Patent document 2 describes that a resin composition having a ratio between a modified block copolymer and a polar thermoplastic polymer in the range of 98:2 to 50:50 is useful as an improved composition containing a modified block copolymer, and use of a resin composition having a ratio between a modified block copolymer and a polar thermoplastic polymer in the range of 2:98 to 50:50 improves the impact resistance of the polar thermoplastic polymer. Specifically, Patent document 2 discloses that a resin composition containing EVOH resin and a modified block copolymer with a ratio of 90:10 to 75:25 provides remarkably improved Izod impact strength, as compared with EVOH resin (see Table 5).
Further, Japanese Unexamined Patent Publication No. Hei 7-173348 (Patent document 3) discloses a resin composition containing EVOH resin and a hydrogenated terpene resin (example 1) or a petroleum-derived hydrocarbon resin (example 2); and also discloses a resin composition further containing a polyamide-based thermoplastic elastomer in addition thereto. However, Patent document 3 discloses an object of enhancing heat seal strength by containing a terpene resin used as a tackifier or a petroleum-based resin having from 5 to 10 carbon atoms in the content of about 0.5 to 10% by weight, and does not disclose an evaluation of an influence and an effect regarding flex crack resistance.
Japanese Unexamined Patent Publication No. 2004-189916 (Patent document 4) discloses a resin composition containing an EVOH resin, a styrene-based block copolymer such as SBS, SIS, SEBS, an unsaturated carboxylic acid-modified block copolymer, and a rubber softener.
The rubber softener contains paraffin oil as a component for softening or plasticizing a rubber. Further, the unsaturated carboxylic acid-modified block copolymer contains a component for improving gas-barrier properties.
Patent document 4 discloses adjusting the component ratio in a predetermined range, in the aspect of securing balance between gas-barrier properties and flexibility of a composition, and securing mechanical properties and gas-barrier properties of a composition.
In Patent document 4, JIS A hardness and tensile rupture strength are measured and evaluated as mechanical properties, but flex crack resistance is not evaluated.
Japanese Unexamined Patent Publication No. 2000-248136 (Patent document 5) proposes, as an example of EVOH resin composition having improved flex crack resistance, a resin composition containing an EVOH resin, a polyamide resin, an olefin-unsaturated carboxylic acid copolymer, and a thermoplastic resin (an ethylene-propylene copolymer, or a rubber such as butadiene rubber) whose elastic modulus at 20° C. is 100 kgf/cm2 or less, with a predetermined ratio. Patent document 5 describes the above resin composition improves gas-barrier properties, transparency (haze value), and flex crack resistance.
Japanese Unexamined Patent Publication No. Hei 8-165397 (Patent document 6) discloses a resin composition containing EVOH resin, and as additives, a polyolefin-based resin and a carboxylic acid-modified polyolefin-based resin. Patent document 6 describes that a resin composition containing EVOH resin and a polyolefin-based resin improves in bending fatigue resistance and impact resistance, however, the resin composition has a problem that physical properties are varied depending on a site where a molded film is used (see paragraph 0005). The Patent document 6 proposes setting the energy value at the extrusion processing in a predetermined range to solve the problem.    [Patent document 1] JP Hei 10-87923A (U.S. Pat. No. 5,895,797)    [Patent document 2] JP Sho 63-304043A    [Patent document 3] JP Hei 7-173348A    [Patent document 4] JP2004-189916A (U.S. Pat. No. 7,354,654)    [Patent document 5] JP2000-248136A    [Patent document 6] JP Hei 8-165397A